Polyimides have been recognized as polymers of choice for passivation and interlayer dielectric in microelectronics for improving the electrical performance characteristics of high density IC devices partly because the organic polymer based insulators have lower dielectric constant compared to the commonly employed inorganic dielectric materials. Use of polyimide insulator also provides process simplification and advantages of having a wide selection of material properties available to meet specific requirements. In high density integrated circuits, faster signal propagation and low cross-talk levels are among some of the important considerations and thus polyimides which have relatively lower dielectric constant and which exhibit many excellent thermal and mechanical properties would be preferred over inorganic dielectrics.
However, the conventional polyimides derived from non-linear aromatic diamine and/or dianhydride precursors such as pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA), 3,3'-4,4'-biphenicdianhydride-4,4'-oxydianiline (BPDA-ODA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride-4,4'-oxydianiline (BTDA-ODA), or 3,3',4,4'-benzophenone tetracarboxylic dianhydride-bis aminophenoxy benzene-3-amino-phenyl-acetylene (BTDA-APB-APA) (THERMID*-Isoimide type--trademark of National Starch and Chemical Co.) have relatively high thermal coefficient of expansion (TCE), typically in the range of 30-60 ppm deg.sup.-1 compared to 2-3 ppm deg.sup.-1 in the case of inorganic materials (non-metallics) used in devices, substrates or packages such as silicon, ceramic, silicon oxide and silicon nitride, and 5-25 ppm deg.sup.-1 in the case of commonly employed metallurgy. This TCE mismatch between the polyimide insulator and inorganic materials in device and packaging thin film multilayer structures results in the development of thermal stresses during high temperature processes in the fabrication cycle resulting in occasional problems of film cracking/delamination which presents a concern as to the performance reliability of end product. Yet another problem with the flexible chain polyimides is excessive swelling in polar solvents such as N-methylpyrrolidone (NMP) which can be a source of stress concentration at critical geometries such as sharp corners resulting in cracking/crazing phenomenon. The polyimide swelling has been found to be highly detrimental during thin film fabrication process especially when a non-compliant, rigid layer such as silicon nitride or silicon oxide is in contact with the underlying polyimide. It has been observed that in such cases, the inorganic film cracks due to excessive swelling of the polyimide underneath causing replication of the crack pattern into the polyimide as it has been observed after the silicon nitride is removed. Also, the flexible chain polyimides show relatively higher moisture uptake (up to 2-3%) which results in performance degradation, which may cause metal/insulator delamination and contributes to corrosion of contacting metallurgy.
Because of such limitations in the use of commonly known polyimides, there has been a great deal of interest recently in the development of low TCE/low stress polyimides for the purpose of generating stress-free multilevel interconnect structures in microelectronics in addition to other applications of these materials.
Y. Misawa et al., IEEE Transactions on Electron Devices, Vol. ED-34, No. 3, March 1987, describe a multilevel interconnection system for submicron VLSI's using multilayered dielectric of plasma silicon oxide and low thermal expansion polyimide, PIQ-L100, which is used in conjunction with plasma CVD SiO.sub.2. For surface planarization, an etch-back technique is described. The low TCE polyimide PIQ-L100 used in this application has been described to have the properties as listed in Table 1 in comparison to a flexible chain polyimide.
U.S. Pat. No. 4,690,999 (Hitachi) discloses uniaxially oriented low thermal expansion polyimides with special properties and a composite shaped article using the same.
TABLE 1 ______________________________________ Low TCE-PI Conv-PI Property Unit (PIQ L100) (PIQ) ______________________________________ Density (g/cm.sup.2) 1.47 1.38 Coefficient of (.times. 10-5 K-1) 0.3 4.5 Thermal Expansion Tensile Strength (Kg/cm) 39 13 Tensile Elongation (%) 22 30 Young's Modulus (Kg/cm.sup.2) 1100 330 Decomposition (.degree. C.) 510 440 Temperature in Air Activation Energy (kcal/Mol) 50 35 of Thermal Decomposition in Air Absorbed Moisture (%) 1.3 2.3 Content ______________________________________
U.S. Pat. No. 4,880,684 (IBM) discloses multilevel structure using glass-ceramic substrate which has capture pads on both sides to provide hermiticity. Multiple layers of flexible chain polyimides PMDA-ODA and BTDA-APB and interconnection metallurgy are formed on glass-ceramic such that capture pads formed on both sides of substrate are in alignment with the substrate vias and thru polyimide overlayers with the interconnection metallurgy between the underlying pad metallurgy, device chip, or pin bonded to surface of the layer.
U.S. Pat. No. 4,789,648 (IBM) discloses a method of producing multilevel metal/polyimide insulator films on a substrate and forming patterned conductive lines simultaneously with stud vias.
In the use of flexible chain polyimide insulators in microelectronics fabrication, it has been recognized that due to large differences in thermal expansion, there is fundamental incompatibility of these polymers with the inorganic and metal contacting materials in device structures. Therefore, frequent problems of cracking/crazing and/or delamination are encountered during the fabrication cycle or when the finished device is subjected to reliability testing conditions involving thermal excursions and T/H (temperature/humidity) or solvent exposures. From consideration of product performance including interface integrity at all levels of processing and end use, it is important that the polyimide insulator has the optimum functional characteristics in terms of thermal expansion match, thermal stability, mechanical properties, adhesion with contacting layers, lower dielectric constant, lower moisture uptake, minimal swelling in NMP, and other desirable features. In addition, successful fabrication of multilevel-metal structure using polyimides in conjunction with contacting materials of diverse properties requires that optimum set of process conditions be developed to provide problem-free fabrication and consistent product performance.